(a) Field of the Invention
The present invention relates to an active matrix liquid crystal display (LCD) device, and more particularly, to an active matrix LCD device wherein liquid crystal is driven by an electric field acting in the direction substantially parallel to the substrates.
(b) Description of the Related Art
LCD devices are generally categorized into two types including a passive matrix LCD device and an active matrix LCD device based on the driving system therefor. The active matrix LCD device includes an active drive element such as thin film transistor (TFT) or diode in each pixel element, for charging a capacitor thereof with a signal voltage while selecting the pixel element for an on-state in a time-division scheme. The capacitor holds the signal voltage during a subsequent off-state of the drive element for displaying an image for the pixel. Compared to the passive matrix LCD device wherein the signal voltage is applied to the liquid crystal (LC) by using a time-division matrix-drive scheme, the active matrix LCD features a higher contrast and a larger screen.
A twisted-nematic mode (referred to as TN mode, hereinafter) has been generally used as the operational mode of the LC in the active matrix LCD device, wherein the aligned direction (referred to as director, hereinafter) of the longer axes of the LC molecules is twisted by about 90 degrees between the transparent substrates, using an electric field in the direction perpendicular to the substrates for rotating the director in the vertical direction.
The TN mode LCD device generally has a defect wherein the view angle for the LCD panel is narrow, that is, the image on the LCD panel has a large view angle dependency, especially in the case of a large screen LCD panel.
For solving the problem view angle dependency, an in-plane switching mode (referred to as IPS mode, hereinafter) has been developed for generating an electric field in the direction parallel to the substrates for rotating the director within the horizontal plane. In the proposed IPS mode LCD, the horizontal alignment of the LC orientation effected by the electric field acting parallel to the substrates affords an advantage in that the double refraction characteristic of the LC is scarcely changed even if the viewpoint is moved, thereby achieving a wider view angle compared to the TN mode LCD device.
FIG. 1 shows a top plan view of a first conventional example of IPS mode active matrix LCD devices, FIGS. 2 and 3 show sectional views taken along line IIxe2x80x94II and IIIxe2x80x94III, respectively, in FIG. 1, and FIG. 4 shows a timing chart of the potentials of electrodes and lines in the first conventional active matrix LCD device.
The first conventional LCD device has a plurality of pixel elements each including a TFT 16 and a pixel electrode 17, a plurality of scanning lines 13 disposed for respective rows of the pixel elements, a plurality of signal lines 14 disposed for respective columns of the pixel elements, and a common electrode 15, which are formed on a transparent insulating substrate 11 (referred to as TFT substrate, hereinafter). Each pixel electrode 17 and a corresponding portion the common electrode 15 extend parallel to each other to generate an electric field 100 having a main component extending substantially perpendicular to the stripe electrodes 17 and 15.
As shown in FIG. 3, the active matrix LCD device further includes a transparent counter substrate 12 disposed in opposed relationship with the TFT substrate 11 with an intervention of a LC layer 18. The counter substrate 12 mounts thereon a black matrix 19, a color filter 20 and a LC orientation layer 21 on respective sides thereof.
In the IPS mode LCD device of FIG. 1, the electric field generated therein includes unnecessary components between the signal line 14 and adjacent electrodes, which necessitates a light shield for covering the unnecessary image component. In the illustrated LCD device, the black mat 19 formed on the counter substrate 12 acts as the shield layer for the space between the signal line 14 and the adjacent electrodes (such as common electrode 15). The arrangement of the TFT substrate 11 with respect to the counter substrate 12 generally involves an alignment error of about 7 to 10 xcexcm after bonding thereof. For assuring the effective light shield, the edge of the black matrix 19 should be located with a margin, such as designated by reference numeral 200 in FIG. 3, from the edge of the common electrode 15. This causes a larger area of the black matrix 19 and a smaller opening rate of the pixel area in the LCD device. Patent publication JP-A-9-80415 proposes for solving the problem low opening rate.
FIG. 5 shows a top plan view of the proposed LCD device, or second conventional LCD device, FIG. 6 shows a sectional view taken along line VIxe2x80x94VI in FIG. 5, and FIG. 7 shows a timing chart in the second conventional LCD device.
In the second conventional LCD device shown in FIG. 5, the signal line 14 partly overlaps with the common electrode 15 as viewed in the direction perpendicular to the substrates. This affords an advantage in that the portions of the black matrix 19 extending in the direction parallel to the signal lines 14 can be omitted together with their margins, and it is sufficient that the black matrix 19 has a portion extending parallel to the scanning lines 13 in the display panel. This achieves a larger opening rate of the pixel area in the LCD device.
In the second conventional LCD devices, however, the black matrix 19 formed on the counter substrate 12 involves another problem, as described hereinafter.
In case of the TN mode active matrix LCD device, since the electric field from the black matrix 19 is shielded by the transparent common electrode 15 formed on the substantially entire surface of the counter substrate 12, the electric potential of the black matrix 19 does not affect the image quality on the LCD panel. However, in case of the IPS mode active matrix LCD device, since the black matrix 19 does not have a shield electrode such as the common electrode between the black matrix 19 and the LC layer 18 in the TN mode active matrix LCD device, the potential of the black matrix 19 fluctuates and thereby affects the image quality on the LCD panel.
It is first noted that the electric potential of the black matrix 19 is not fixed in the IPS mode active matrix LCD device, whereas the black matrix 19 is implemented by materials of a high electric conductivity, such as black resist wherein a metal or carbon black is dispersed. Thus, the potential of the black matrix 19 is generally determined based on the capacitive coupling acting between the same and the signal lines 14, the scanning lines 13 and the common electrode 15
Assuming that the potential of the black matrix 19, the voltages of the signal lines 14, the scanning lines 13, and the common electrode 15, the coupling capacitances between the black matrix 19 and the signal line 14, between the black matrix 19 and the scanning line 13, between the black matrix 19 and the common electrode 15 are represented by Vbm, Vd, Vg, Vcom, Cbm-d, Cbm-g, Cbm-com, respectively, the potential Vbm of the black matrix is expressed by equations 1 and 2:
Vbm=Vdxc3x97Cbm-d/Ctotal+Vgxc3x97Cbm-g/Ctotal+Vcomxc3x97Cbm-com/Ctotalxe2x80x83xe2x80x83(1) 
Ctotal=Cbm-d+Cbm-g+Cbm-comxe2x80x83xe2x80x83(2) 
Voltage Vg of the scanning line 13 is higher than signal voltage Vd of the signal line 14 and the voltage of the pixel electrode 17 during the selected small time interval when the TFT is turned on by voltage Vg of the scanning line 13, and is lower than voltage Vd and the voltage of the pixel electrode 17 during the remaining time interval.
Voltage Vd of the signal line 14 changes at an interval of the horizontal scanning cycle to charge the pixel electrodes selected in succession to a desired voltage. Voltage Vcom of the common electrode 15 also changes at an interval similar to the interval of voltage Vd or keeps a constant voltage, depending on the drive system in the LCD device. Thus, it will be understood from the equations (1) and (2) that the black matrix has a potential different from the potential of the common electrode and changes at a different interval.
The potential of the black matrix 19 deteriorates the image quality as follows. The desired electric field 100 generated between the pixel electrode 17 and the common electrode 15 reverses its polarity at a small time interval such as frame interval to prevent a defective image such as burning or stain on the display panel by preventing localization of impurity ions or charge-up of the insulator.
In the conventional LCD devices shown in FIGS. 1 and 5, the potential of the black matrix 19 is different from the potential of the pixel electrode 17 and the potential of the common electrode 15, and changes at a different time interval. This causes that a deleterious stray electric fields 101 and 102 act in the pixel area 300, which deteriorates the image quality on the display panel. In addition, the different timing and magnitude of the electric fields causes an effective DC component in the electric field in the display panel, which generates burning or stain on the panel.
Referring to FIG. 4 showing results of simulation, wherein a voltage of 5 volts is applied between the pixel electrode 17 and the common electrode 15, assuming that 480 scanning lines 13 are provided in the LCD device with a dot-inversion drive system wherein pixel electrodes in the adjacent pixels have opposite potential polarities with respect to the common electrode. Voltage Vg of the scanning lines 13 is 21 volts and xe2x88x928 volts during an on-state and an off-state thereof, respectively, voltage Vd of the signal lines 14 is 12 volts and 2 volts during a positive voltage frame peirod and a negative voltage frame period, respectively, and voltage Vcom of the common electrode 15 is fixed at 6 volts.
When the scanning line 13 is off, voltage of the pixel electrode 17 falls below voltage Vd of the signal line 14 due to the charge in the TFT channel flowing into the pixel electrode 17 and to the coupling capacitance between the scanning line 13 and the pixel electrode 17. The voltage difference between the pixel electrode 17 and the signal line 14 is about 1 volt in the simulation, which causes the voltage difference between the pixel electrode 17 and the common electrode at 5 about volts. The black matrix 19 is disposed to cover the signal lines 14 and the scanning lines 13, and had an initial potential of zero volt.
As understood from of FIG. 4, potential Vbm of the black matrix 19 fluctuates around about 3V during the time interval when the pixel electrode 17 maintains off voltage. The electric field generated by potential Vbm of the black matrix 19 enters the pixel area due to the arrangement of the black matrix 19.
In FIG. 1, electric fields 101 and 102 generated by the potential of the black matrix 19 are also illustrated. In the positive voltage frame period, as shown in FIG. 4, voltage Vcom of the common electrode 15 is 6 volts, voltage of the pixel electrode 17 is 11 volts, voltage Vbm of the black matrix 19 is about 3 volts. Thus, a potential difference of 5 volts is generated between the common electrode 15 and the pixel electrode 17, and generates the desired parallel electric field 100. A potential difference of about 3 volts is also generated between the black matrix 19 and the common electrode 15 in the vicinity of the black matrix 19 to generate electric field 101. A potential difference of about 8 volts is further generated between the black matrix 19 and the pixel electrode 17 to generate electric field 102.
Electric fields 101 and 102 other than the desired parallel electric field 100 generated in the vicinity of the black matrix 19 degrade the image quality in the display panel. In a negative voltage frame period, a potential difference of 5 volts having a polarity opposite to the polarity in the positive voltage frame period is generated between the common electrode 15 and the pixel electrode 17, to generate the desired parallel electric field 100. In the vicinity of the black matrix 19, however, a potential difference of about 3 volts having a polarity similar to that in the positive voltage frame period is also generated between the black matrix 19 and the common electrode 15 to generate electric field 101. The electric field having the same polarity both in the positive voltage frame period and in the negative voltage frame period means an effective DC bias voltage being applied to the black matrix 19. The effective DC bias voltage causes a defect such as burning or stain to deteriorate the image quality in the display panel.
FIG. 7 shows results of simulation in the second conventional LCD device, with conditions similar to those in the simulation shown in FIG. 4. In the simulation of FIG. 7, the absence of a portion of the black matrix 19 overlapping the signal lines 14 causes a smaller capacitance Cbm-d between the black matrix 19 and the signal line 14 expressed in equations (1) and (2). Thus, voltage Vbm of the black matrix 19 is scarcely affected by voltage Vd of the signal lines 14. Since voltage Vd of the signal lines 14 in root-mean-square value is substantially equal to voltage Vcom of the common electrode 15, a larger difference is generated between voltage Vbm of the black matrix 19 and voltage Vcom of the common electrode 15.
Accordingly, electric field 101 generated between the black matrix 19 and the pixel electrode 17 as well as electric field 102 generated between the black matrix 19 and the common electrode 15 is larger in this case and generates a larger DC component compared to the first conventional LCD device.
In short, in the conventional IPS mode active matrix LCD devices, the potential of the black matrix 19 formed on the counter substrate 12 generates undesired electric fields in the display panel to degrade the image quality thereon.
It is therefore an object of the present invention to provide an IPS mode active matrix LCD device capable of providing a high image quality by suppressing the undesired electric fields.
The present invention provides an active matrix LCD device comprising first and second transparent substrates, liquid crystal sandwiched between the first transparent substrate and the second transparent substrate, the first transparent substrate including a plurality of pixel elements arranged in a matrix, each of the pixel elements defining a pixel area for image display and having a thin film transistor (TFT) and a pixel electrode connected to a drain of the TFT, a plurality of scanning lines each connected to gates of the TFTs in a corresponding row of the pixel elements, a plurality of signal lines each connected to a source of the TFTs in a corresponding column of the pixel elements, and a common electrode having an electrode portion disposed for each of the pixel elements in association with the pixel electrode, one of the first transparent substrate and the second transparent substrate including a black matrix for covering an area other than the pixel area as viewed in a first direction perpendicular to the first and second substrates, the electrode portion of the common electrode encircling the pixel area as viewed in the first direction.
In accordance with the active matrix LCD device of the present invention, the pixel area is shielded by the common electrode encircling the pixel area against a stray electric field from the black matrix, thereby improving the image quality of the active matrix LCD device.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.